Steel wire rod for hard drawn spring, drawn wire rod for hard drawn spring and hard drawn spring, and method for producing hard drawn spring

ABSTRACT

Disclosed is a steel wire rod for hard-drawn springs capable of exhibiting fatigue strength and sag resistance equivalent to or higher than springs made of an oil-tempered wire. The steel wire rod contains carbon in a range from 0.5 to less than 0.7 mass %, silicon in a range from 1.4 to 2.5 mass %, manganese in a range from 0.5 to 1.5 mass %, chromium in a range from 0.05 to 2.0 mass %, and vanadium in a range from 0.05 to 0.40 mass %, and has an area ratio Rp with respect to pearlite which satisfies the mathematical expression (1):
 
 Rp (area %)≧55×[ C ]+61  (1)
 
where [C] denotes the content (mass %) of carbon.

TECHNICAL FIELD

This invention relates to a steel wire rod for hard-drawn springs whichis useful as a material for valve springs, clutch springs, brakesprings, etc. of automotive engines, a wire for hard-drawn springs usingsuch a wire rod, hard-drawn springs, and a useful method for producingsuch hard-drawn springs.

BACKGROUND ART

As development of light-weighted construction and high performance forautomotive vehicles, etc. has progressed, high stress design has beenrequired for valve springs, clutch springs, brake springs or the like.Springs excellent in fatigue strength and sag resistance have beendemanded. In particular, there is a strong demand for high stress designof valve springs.

Recently, it has been a custom that valve springs are primarily producedby cold coiling an oil-tempered wire that has been applied withquenching and tempering. According to the Japanese Industry Standards(JIS), for example, an oil-tempered wire (according to JIS G3561) forvalve springs is separately defined from an ordinary oil-tempered wire(according to JIS G3560). Thus, it is required to strictly control thekind of steel, allowable impurity content, depth of flaw, etc.

The oil-tempered wire has the following advantage and disadvantage. Asregards the advantage, since the oil-tempered wire has temperedmartensite, it can provide springs of high strength, and it hasexcellent fatigue strength and sag resistance. As regards thedisadvantage, there is required a large-scaled facility and cost forheat treatment such as quenching and tempering to produce theoil-tempered wire.

Some of valve springs of low load stress are obtained by drawing carbonsteel that has ferrite/pearlite or pearlite to increase strength (alsocalled “hard-drawn wire”), and by cold coiling the hard-drawn wire.According to the JIS, such a wire belongs to the criteria of “Piano WireType V” for “valve springs or like springs” in the criteria of pianowires according to JIS G3522.

Springs made of the aforementioned hard-drawn wire (hereinafter,referred to as “hard-drawn springs”) can be produced with a low costbecause heat treatment is not required in the production method.However, the wire in which ferrite/pearlite or pearlite has beensubjected to drawing has low fatigue properties and low sag resistance.Accordingly, even if such a wire is used for springs, it cannot providefor high-strength springs that are required in the recent technology.

There also have been studied various techniques to produce high-strengthhard-drawn springs in light of the advantage of low-cost production.Japanese Unexamined Patent Publication No. HEI 11-199981 proposes anexemplified method for obtaining cementite of a specific configurationby performing a wire drawing process to pearlite ineutectoid-hypereutectoid steel, which is usable as a piano wire havingproperties equivalent to an austempered wire. This method, however,unavoidably raises the production cost because the production process iscomplicated such that a step of changing the wire drawing direction isadditionally required.

In view of the above, an object of this invention is to provide a steelwire rod used for producing hard-drawn springs capable of exhibitingfatigue strength and sag resistance equivalent to or higher than springsproduced by an oil-tempered wire, a wire for hard-drawn springs, suchhard-drawn springs, and a useful method for producing such hard-drawnsprings with a low cost.

DISCLOSURE OF THE INVENTION

An inventive steel wire rod for hard-drawn springs that has accomplishedthe above object contains carbon in a range from 0.5 to less than 0.7mass %, silicon in a range from 1.4 to 2.5 mass %, manganese in a rangefrom 0.5 to 1.5 mass %, chromium in a range from 0.05 to 2.0 mass %, andvanadium in a range from 0.05 to 0.40 mass %, and has an area ratio Rpwith respect to pearlite which satisfies the mathematical expression(1):Rp(area %)≧55×[C]+61  (1)where [C] denotes the content (mass %) of carbon.

It is effective for the inventive steel wire rod (a) to contain nickelin a range from 0.05 to 0.5 mass % or (b) to satisfy the requirementthat the number of carbide and carbo-nitride of vanadium and chromium,complex carbide and complex carbo-nitride of vanadium and chromium eachhaving a diameter of 50 nm or less in terms of a hypothetical circle, isnot smaller than ten per unit area of μm² in lamellar ferrite. With thisarrangement, the properties of the hard-drawn springs can be furtherimproved.

An inventive wire for hard-drawn springs that has accomplished the aboveobject contains carbon in a range from 0.5 to less than 0.7 mass %,silicon in a range from 1.4 to 2.5 mass %, manganese in a range from 0.5to 1.5 mass %, chromium in a range from 0.05 to 2.0 mass %, and vanadiumin a range from 0.05 to 0.40 mass %, and has an area ratio Rp withrespect to pearlite which satisfies the mathematical expression (1), andhas a tensile strength TS which satisfies the mathematical expression(2):Rp(area %)≧55×[C]+61  (1)

-   -   where [C] denotes the content (mass %) of carbon,        −13.1d ³+160d ²−671d+3200≧TS≧−13.1d ³+160d ²−671d+2800  (2)

where d is a diameter (mm) of the wire which satisfies the expression[1.0≦d≦10.0].

It is effective for the wire to contain nickel in a range from 0.05 to0.5 mass %.

High-strength hard-drawn springs are producible by using the above steelwire rod or the above wire. Further, preferably, the hard-drawn springssatisfy the requirement that a residual stress of the spring is changedfrom a compression to a tension at a depth of 0.05 mm or more from theinner surface of the spring. More preferably, the depth-wise position ofthe spring is 0.15 mm or more from the inner surface of the spring.Furthermore, it is effective to apply a nitriding process on thehard-drawn springs.

In producing the aforementioned hard-drawn springs, it is desirable toapply a stress τ (MPa) to the springs at a temperature not lower than aroom temperature at least once after shot-peening, wherein the stress τsatisfies the mathematical expression (3):τ≧TS(MPa)×0.5   (3)

where TS denotes a tensile strength of the wire. In this productionmethod, preferably, the temperature at which the stress τ is applied is120° C. or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between carbon content and arearatio of pearlite in comparison of the inventive steel wire rod withordinary carbon steel wire rod.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors of this application have made extensive study andresearches in order to find out a steel material for hard-drawn springsthat can accomplish the above object of this invention from a variety ofangles. As a result of their study, they have found that a steel wirerod whose chemical composition has been strictly defined and whichsatisfies the range [range as defined in the aforementioned mathematicalexpression (1)] in terms of a relationship between the area ratio ofpearlite and the carbon content in the steel wire rod can producehard-drawn springs which exhibit fatigue strength and sag resistanceequivalent to or higher than springs using an oil-tempered wire, andaccomplished this invention.

The chemical composition of the inventive steel wire rod is required tobe properly regulated in the following manner. The reasons for settingthe range of each component are as follows.

C: 0.5 to Less than 0.7 Mass %

Carbon is a useful element to enhance tensile strength of a wire and tosecure certain fatigue properties and sag resistance of springs. Anordinary piano wire contains carbon of about 0.8 mass %. However, inproducing a wire of high strength, which is an object of this invention,if the content of carbon is 0.7 mass % or larger, a wire containingcarbon in such a high content has increased defect sensitivity, and acrack occurs from surface flaws and inclusion. As a result, fatigue lifeof springs made of such a wire may be shortened. In view of this, inthis invention, the content of carbon is set to be less than 0.7 mass %.On the other hand, if the content of carbon is less than 0.5 mass %, awire containing carbon in such a low content cannot provide a tensilestrength as required for producing high-strength springs. To makematters worse, in case of such a low carsbon content, the content ofpro-eutectoid ferrite, which promotes generation of cracks due tofatigue, may increase, thereby deteriorating fatigue properties ofsprings. In view of this, it is necessary to set the lower limit ofcarbon content at 0.5 mass %.

Si: 1.4 to 2.5 Mass %

Silicon is an element which enhances a tensile strength of a wire by asolid solution strengthening and which contributes to improvement infatigue properties and sag resistance of springs. It is necessary toincrease the content of silicon by an amount corresponding to thelowered content of carbon. In view of this, the lower limit of siliconcontent is set at 1.4 mass %. On the other hand, if the silicon contentexceeds 2.5 mass %, deoxidation, flaws or the like may increase on thespring surface, which may lower the fatigue strength of springs.Preferably, the lower limit of the silicon content is set at about 1.7mass %, and the upper limit thereof is set at about 2.2 mass %.

Mn: 0.5 to 1.5 Mass %

Manganese is an element which makes pearlite in fine and orderly mannerand which contributes to improvement in fatigue properties of springs.In order to allow springs to exhibit such effects, it is required to setthe content of manganese at 0.5 mass % or more. However, an excessivecontent of manganese may likely to generate bainite at the time of hotrolling or patenting, which resultantly may deteriorate fatigueproperties of springs. In view of this, the manganese content isrequired to be set at 1.5 mass % or lower. Preferably, the lower limitof the manganese content is set at about 0.7 mass %, and the upper limitthereof is set at about 1.0 mass %.

Cr: 0.05 to 2.0 Mass %

Chromium is an element which is useful in narrowing pearlite lamellarspacing, increasing strength after hot rolling or heat treatment, and inimproving sag resistance of springs. In order to allow springs toexhibit the above effects, it is required to set the content of chromiumat 0.05 mass % or more. An excessive content of chromium, however, mayundesirably extend the period for patenting, and deteriorate toughnessand ductility of springs. In view of this, it is required to set thechromium content at 2.0 mass % or less.

V: 0.05 to 0.40 Mass %

Vanadium is an element which is useful to fine the nodule size ofpearlite and in improving wire drawability, toughness and sag resistanceof springs, etc. In order to allow springs to exhibit the above effects,it is required to set the vanadium content at 0.05 mass % or more.Preferably, the vanadium content is set at 0.10 mass % or more.Excessive content of vanadium, namely, vanadium content of more than0.40 mass % may likely to produce bainite at the time of hot rolling orpatenting, which may resultantly shorten fatigue life of the springs.

The basic chemical composition of the inventive steel wire rod is asmentioned above. It is possible to add nickel in the range from 0.05 to0.5 mass % according to needs. The range of nickel content and thereason for setting the range thereof are as follows.

Ni: 0.05 to 0.5 Mass %

Nickel is an element which is effective in lowering defect sensitivity,increasing toughness of springs, suppressing breakage trouble at thetime of coiling, and in improving fatigue life of springs. In order toallow nickel to exhibit the above effects, it is preferable to containnickel of 0.05 mass % or more. Excessive content of nickel, however, maylikely to produce bainite at the time of hot rolling or patenting, whichresultantly may produce disadvantages rather than the above advantages.In view of this, the nickel content is preferably set at 0.5 mass % orlower. Preferably, the lower limit of nickel content is set at 0.15 mass%, and the upper limit thereof is set at 0.30 mass %.

The essential component of the remainder constituting the inventivesteel wire rod other than the aforementioned chemical components isiron. However, it should be appreciated that minor components other thanthe above components may be contained as far as they do not interferethe properties of the steel material for producing the inventivesprings. In other words, a steel wire rod containing such minorcomponents falls in the scope of this invention. Some of the exemplifiedminor components are molybdenum in the content of about 0.5 mass % orless, which is added for the purpose of obtaining improved effect due tohardenability, aluminum in the content of about 0.05 mass % or less,which is added as a deoxidizer at the time of producing steel, andimpurities, particularly, unavoidable impurities such as phosphorous,sulfur, arsenic, antimony, tin, etc. (e.g., 0.02% or less with respectto phosphorous or sulfur, and 0.01% or less with respect to arsenic,antimony, and tin).

It is required to set the composition of the inventive steel wire rod inan appropriate range [range as defined in the aforementionedmathematical expression (1)] in light of a relationship between the arearatio of pearlite and the carbon content in a steel wire rod. The reasonfor setting the range is as follows.

The carbon content of the steel used in this invention is required to beset not lower than 0.5 mass % to less than 0.7 mass % as mentionedabove, namely, lower than the content of eutectoid component. When awire rod is produced by using a steel having such a compositionaccording to a conventional method, pro-eutectoid ferrite is likely togenerate on the wire rod, and fatigue failure may occur from suchpro-eutectoid ferrite, thus shortening fatigue life of resultantsprings. In order to eliminate such a drawback, it is required tosuppress generation of pro-eutectoid ferrite as much as possible whileincreasing the ratio of pearlite.

FIG. 1 is a graph showing a relationship between the carbon content andthe area ratio of pearlite in a steel material. Whereas generallyavailable carbon steel has a relatively low pearlite area ratio, theinventive steel wire rod has a relatively high pearlite area ratioconsidering its relation to the carbon content in view of the abovepoint.

It is effective to maximize the cooling rate of the wire rod in atemperature zone at least covering a transformation point A_(e3)(uppermost temperature at which austenite and ferrite can coexist in anequilibrium state) and a transformation point A_(e1) (uppermosttemperature at which ferrite and cementite can coexist) at the time ofhot rolling or patenting in order to obtain such a structure thatsatisfies the aforementioned mathematical expression (1). Specifically,in case of hot rolling, it is effective to control the cooling rate at5° C./sec or higher, preferably at 10° C./sec or higher in the abovetemperature zone as a cooling condition on a conveyor. It should benoted, however, that excessively continuing the cooling may fail toobtain fine pearlite, with the result that a structure such as bainiteas a result of super cooling may be intruded, thereby lowering toughnessof the resultant springs. In view of the above, it is recommended tomonitor the cooling condition each time the wire rod is carried over theconveyor at certain positions in such a manner that the cooling isgradually carried out until the temperature of the wire rod falls belowabout 550° C.

In case of patenting, the cooling rate from the transformation pointA_(e3) to the transformation point A_(e1) is relatively fast. However,it is preferable to select a medium having large heat conductivity forperforming isothermal transformation of the wire rod. Specifically, itis preferable to use a lead bath or a salt bath rather than a fluidbath. It is preferable to provide a cooling step between a step ofaustenitizing in a heating furnace and a step of performing isothermaltransformation in a furnace to thereby forcibly perform cooling by thecooling step in order to expedite cooling operation. It is alsoeffective to maximize the wire speed in order to accelerate the coolingrate. It is also possible to measure the pearlite area ratio Rp withrespect to a wire or with respect to springs as a final product becausethe pearlite area ratio Rp does not greatly vary depending on a statusas to whether a wire drawing process or a spring forming process whichfollows the wire drawing process has been implemented or not.

It is desirable to reinforce a ferrite part of a steel wire rod, whichis the weakest part of pearlite since such reinforcement can improve sagresistance of resultant springs. To reinforce ferrite means toprecipitate micro precipitants in the ferrite to such an extent that thesum of the number of precipitants, namely, carbide and carbonitride ofvanadium and chromium, complex carbide and complex carbonitride ofvanadium and chromium (hereinafter, all these kinds of precipitants aresometimes referred to as “complex carbonitride, etc.”) in which eachprecipitant has a diameter of 50 nm or less in terms of a hypotheticalcircle, is ten or more per μm². The term “diameter in terms of ahypothetical circle” means a diameter of a hypothetical circle whosearea is equivalent to the actual area of a corresponding precipitant.

As far as the inventive steel wire rod satisfies the above requirements,it is allowable for the inventive steel wire rod to partially containcomplex carbide, etc. having a diameter larger than 50 nm in terms of ahypothetical circle. However, it is preferable that almost or all thecomplex carbide etc. has a diameter of 50 nm or smaller. The lower limitof the size of such complex carbides is not specifically limited.However, considering the fact that the maximal size of a substancerecognizable by, e.g., a currently available transmission microscope ofmagnification at 150,000 is about 10 nm, it is true to say that about 10nm is substantially a lower limit of a measurable precipitant accordingto a accurate state of art.

It is preferable (1) to cool the wire rod at a cooling rate of 2° C./secor more in a a temperature zone from 900 to 800° C., which is anaustenitizing temperature zone, (namely, not to precipitate in theaustentizing temperature zone) after rolling and then cool the wire rodat a cooling rate from 0.5 to 1.0° C./sec in a temperature of 750 to400° C. or (2) to transform the wire rod at 640° C./sec after heatingthe wire rod to 900° C., and then cool the wire rod at a cooling rate of0.5 to 1.0° C./sec until the temperature of the wire rod reaches 400° C.in order to satisfy the above requirements while securely dispersing alarge amount of micro precipitants in the ferrite.

Drawing the steel wire rod having been treated as mentioned above into awire and coiling the wire enables to produce springs capable ofexhibiting the desirable properties. It is effective to allow the wireobtained by drawing the steel wire rod (hard-drawn wire for springs) tosatisfy the above mathematical expression (2) to allow the resultantsprings to exhibit the above effects more efficiently.

In association with the expression (2), the tensile strength of a wireis defined in accordance with the diameter of a wire in the criteria ofJIS G3522-SWP-V. Specifically, tensile strength TS defined in theafoementioned criteria is lower than that in the criteria of SWP-B, etc.designed for springs of general use. The reason for setting the tensilestrength of the wire according to the above standard lower than that forsprings of general use is conceived as follows. Excessively high tensilestrength may likely to increase defect sensitivity of wire, lowertoughness and ductility of springs, and cause undesirable breaking ofwire while drawing, breakage during coiling, fatigue failure, brittlefracture of springs, etc.

In view of the above, this invention has succeeded in producing springsfrom the wire having the tensile strength TS equal to or larger than thevalue in the right object of the expression (2), as well as use of suchsprings by lowering defect sensitivity and increasing toughness andductility. However, setting the tensile strength TS of the wireexcessively high may cause an adverse effect due to increased defectsensitivity or lowered toughness and ductility. In view of such adrawback, according to this invention, the value in the left object ofthe expression (2) is set as the upper limit of the tensile strength.Although a wire which satisfies the requirements in the expression (2)may be obtainable with use of a conventional wire drawing facility.However, considering a recent demand that a wire of an extremely highstrength is subjected to plastic deformation, it is desirable tooptimally set the requirements so as not to cause breaking of wire. Inview of this, the following matters should be considered: (1) a metallicsoap is used as a lubricant after coating the wire rod with phosphate asa pre-drawing process; (2) the reduction of area of each die used forwire drawing is set in a range from 15 to 25% (the reduction of area ofa die used in a final stage of wire drawing can be set lower than theaforementioned range in order to regulate a residual stress), and (3) awire drawing rate should not be exceedingly raised in order to preventtemperature rise during the drawing, etc.

In the inventive springs, it is preferable that a residual stress ischanged from a compression to a tension at a depth of 0.05 mm or morefrom the inner surface of the spring, and preferably at a depth of 0.15mm or more. Generally, valve springs and high-strength springsequivalent thereto are used in a state where a compressive residualstress is exerted onto the spring surface by shot-peening. When such aresidual stress is measured stepwise in a depth direction of the springfrom the inner surface thereof, the measured residual stress turns froma compression to a tension at a certain depth-wise position(hereinafter, this point is referred to as “crossing point”). Thecrossing point depends on the condition of shot-peening, the hardness ofthe steel material, the residual stress distribution of the basematerial of the springs before shot-peening, etc. A tension residualstress due to wire drawing is exerted to the inner surface of ahard-drawn wire produced according to a conventional method.Accordingly, the depth-wise position of the hard-drawn springs aftershot-peening is likely to be shortened compared to a case of springsmade of an oil-tempered wire. On the other hand, it is desirable toapply a strong force at shot-peening to the inventive hard-drawn springsthan springs made of an oil-tempered wire in such a manner that thedepth-wise position of the spring, namely, the crossing point is set asdeep as at 0.05 mm or more, and preferably at 0.15 mm or moreconsidering that hard-drawn springs made of the inventive wire arerequired to be used under a higher stress than hard-drawn springs ofgeneral use.

In order to set the depth-wise position of the spring, namely, thecrossing point at the aforementioned level, it is effective (a) to setthe reduction of area of a die used in a final stage of drawing at 10%or less, preferably in the range from about 3 to about 6%, (b) to set astress relieving annealing temperature after coiling at 360° C. orhigher, (c) to perform shot-peening at least once with use of a shothaving an average diameter of 0.3 mm or larger, preferably 0.6 mm orlarger for the purpose of reducing a tension residual stress during awire drawing process.

In the case where it is expected that the inventive springs be usedunder a particularly severe and stressful condition, it is effective toapply a nitriding process to the spring surface. Applying a nitridingprocess makes it possible to further improve fatigue strength of thesprings. Such a nitriding process has been conventionally applied tovalve springs made of an oil-tempered wire. However, there has not beenapplied a nitriding process to conventional hard-drawn springs. This isbecause heretofore hard-drawn springs have not been used under such asevere and stressful condition as required in the recent technology, andit has been conceived that a desirable effect cannot be obtained fromthe conventional hard-drawn wires having a conventional chemicalcomposition even if nitriding process is applied on resultant springs.

Applying a hard drawing process to a wire rod having the chemicalcomposition as defined in this invention and applying a nitridingprocess to the hard-drawn springs enables to improve fatigue life of thesprings. The reason why the hard-drawn springs having been applied witha nitriding process exhibits the above effects is conceived as follows.Specifically, since the inventive springs have low carbon content, thevolume of ferrite phase to cementite phase constituting pearlite isincreased. Furthermore, the strength of the inventive wire depends onthe strength of ferrite itself because ferrite has been strengthened byalloy elements such as silicon, vanadium, and chromium. Accordingly, itis conceived that increasing the strength of ferrite by nitriding leadsto direct improvement of fatigue strength of the springs. The inventorsof this application verified that the effect resulting from performing anitriding process is most remarkably obtainable when the nitridingprocess is performed in such a manner that the depth-wise position at 10μm from the surface of the spring has a hardness of HV600 or more(preferably HV700 or more).

It is effective to apply a stress τ to springs at least once at a roomtemperature or higher, preferably at 120° C. or higher aftershot-peening, wherein the stress τ satisfies the above mathematicalexpression (3) when forming the inventive wire rod or the inventive wireinto the springs. Generally, hard-drawn springs have a low sagresistance compared to springs made of an oil-tempered wire. Thisinvention has been made for the purpose of improving sag resistance ofthe hard-drawn springs by setting the chemical compositions of the steelin the respective predetermined ranges and by increasing the tensilestrength of the wire. However, there is a case that a further improvedsag resistance is required depending on the purpose and condition of useof the hard-drawn springs. To cope with such a demand, it is effectiveto apply the stress τ to springs at least once at a room temperature ormore (preferably, 120° C. or more). It is conceived that applying thestress as mentioned above enables to stabilize dislocation accompaniedby wire drawing and to enhance resistance of the wire against plasticdeformation. It should be noted that the tensile strength TS of thehard-drawn wire in the expression (3) is a value measured with respectto the wire.

EXAMPLES

Hereinafter, this invention is described in more details with referenceto the examples. It should be appreciated that this invention is notlimited to the examples, and as far as not departing from the gist ofthis invention, any modification and alteration is embraced in thetechnical scope of this invention.

Steel materials (A to I) having the chemical compositions respectivelyas shown in Table 1 were melted, poured into a mold, and subjected tohot rolling, and steel wire rods each having a diameter of 9.0 mm wereproduced. At this stage, the size of the compounds precipitated in theferrite among the pearlite of each wire rod was measured. The size ofthe compounds was measured by photographing the compounds precipitatedon the wire rod by thin-film replica method with a transmission electronmicroscope (TEM) at an accelerated voltage of 200 KV and magnificationof 150,000. Among the precipitating compounds, counted was the number ofmicro precipitants each having a diameter of 50 nm or less in terms of ahypothetical circle which have been precipitated in the ferrite of 1 μm²[(150 mm²) at the magnification of 150,000]. The site for themeasurement was set at the depth-wise 0.2 mm-position from the surfaceof the wire rod in view of the facts that (a) the surface part of aspring is a part where the spring is exerted with a maximal stress andthat (b) the surface of a roll steel is sliced off by SV process afterrolling. Further, the number of complex carbonitrides, etc. was countedthrough visual recognition by the TEM. The visually non-recognizablemicro complex carbonitrides, etc. have been identified as such complexcarbonitrides, etc. by means of an X-ray diffraction pattern. The numberof complex carbonitirdes, etc. of a size in a range from 10 to 50 nm wascounted by the TEM at a magnification of 150,000. Furthermore, themeasurement was performed with respect to each steel wire rod througharbitrary three different fields of view, and the average of themeasurement results was obtained (see Table 2).

After the hot rolling, softening was performed with respect to all thesteel wire rods except the steel wire rod made of the steel material F.Then, shaving, patenting, and wire drawing were performed with respectto all the steel wire rods. Thus, wires each having the diameter asshown in Table 2 were produced. Patenting was performed by setting anaustenitizing heating temperature at 940° C. and by setting the drawingrate at a relatively high level of 8.0 m/min. Further, with respect toExamples No. 1 to 10, 12, and 15, the wire rods were subjected to rapidcooling by forcibly blowing pressurized air onto the wire rodes beforebeing carried into a lead furnace at a temperature of 620° C. in orderto increase the area ratio of pearlite. All the wire rods of theExamples Nos. 1 to 9, 11, and 12 except the wire rod of Example No. 10were furnace, and then cooled at a cooling rate ranging from 0.5 to 1°C./sec until the temperature of the wire rods has lowered to 400° C. Thewire rod of Example No. 10 was cooled at a cooling rate of 3° C./secuntil the wire rod was cooled to 400° C. after brought into anisothermal treatment in the lead furnace.

The wire drawing was carried out by a continuous wire drawing machineequipped with 8 pieces of dies in which the reduction of area of eachdie except the die used in a final stage of wire drawing was set in therange from 15 to 25%, and the reduction of area of the die of the finaluse was set at 5%, and the wire drawing rate at the die of the final usewas set at 200 m/min. Furthermore, cooling wire drawing was carried outin which the wire rod was directly water-cooled while drawing in orderto prevent temperature rise of the wire rod accompanied by wire drawing.

The thus obtained wires produced by wire drawing were formed intosprings at a room temperature, and subjected to stress relieving (400°C.×20 min.), seat position grinding dual shot-peening, low-temperatureannealing (230° C.×20 min.), and presetting (application with a stress)(τ_(max) corresponding to 1200 MPa). Presetting was performed at about180° C. with respect to the springs of Examples Nos. 4 through 9 byutilizing redundant heat generated from the low-temperature annealing.Nitriding process was applied to the springs of Examples Nos. 5 and 6 at460° C. for 5 consecutive hours. The pearlite area ratio of the springswas analyzed and evaluated by taking photographs of cross sections ofthe wires after patenting by an optical microscope (400 magnifications,10 fields) and by analyzing the photos according to a computerized imageanalyzer.

TABLE 1 Kind of Chemical Composition (mass %) Ex. No. Steel C Si Mn NiCr V 55 × [C] + 61 Rp[%] 1 A 0.59 1.95 0.88 0.23 0.90 0.11 93.5 96.5 2 A0.59 1.95 0.88 0.23 0.90 0.11 93.5 96.5 3 A 0.59 1.95 0.88 0.23 0.900.11 93.5 96.5 4 A 0.59 1.95 0.88 0.23 0.90 0.11 93.5 96.5 5 A 0.59 1.950.88 0.23 0.90 0.11 93.5 96.5 6 A 0.59 1.95 0.88 0.23 0.90 0.11 93.596.5 7 B 0.51 1.80 0.75 0.08 1.09 0.17 89.1 97.7 8 C 0.65 1.91 0.90 0.190.64 0.09 96.8 98.7 9 D 0.68 1.45 0.75 0 0.40 0.25 98.4 99.0 10  E 0.551.89 0.81 0.20 0.15 0.08 91.3 98.7 11  J 0.60 1.97 0.79 0.20 1.75 0.1294.0 97.1 12  K 0.62 1.85 0.71 0.15 0.70 0.34 95.1 99.0 13  A 0.59 1.950.88 0.23 0.90 0.11 93.5 88.6 14  F 0.92 0.25 0.75 0 0 0 — 100.0 15  G0.80 1.90 0.85 0.18 0.85 0.15 — 100.0 16  H 0.80 1.26 0.92 0.32 0.870.20 — 100.0 17  I 0.62 0.96 0.79 0.21 0.96 0.13 95.1 97.9

Fatigue test was performed with respect to the thus obtained springsunder a load stress of 637±588 MPa, and the breaking life was measuredlife was measured with respect to springs under a stress of 882 MPa at120° C. for 48 consecutive hours, and the thus measured residual shearstrain was set as an index for sag resistance (namely, the smaller theresidual shear strain was, the better the sag resistance was).

The results of measurements are shown in Table 2 along with therespective conditions for manufacturing the springs, tensile strength TSof wire, values in the right object and left object in the expression(2), crossing point, hardness at 10 μm depth-wise position from thespring surface, and the number of precipitants.

The hardness at the 10 μm depth-wise position from the spring surfacewas measured by a so-called “Code method” in which a test piece wasembedded in a resin at a known inclination angle, the Vickers hardness(load of 300 g) was measured with respect to the test piece whosesurface was polished, and the thus obtained Vickers hardness wasconverted into a corresponding value in a vertical direction. Theresidual stress was measured according to an X-ray diffraction method.The profiles of residual stress with respect to each spring in thedepth-wise direction were evaluated by removing the surface layers ofthe springs stepwise by chemical polishing and by performing X-raydiffraction measurement.

TABLE 2 Distance to Hardness at Number of Wire TS of Left Right CrossingStress at 10 μm⁻ Precipitants of Residual Fatigue diameter wire Objectin Object in point 120° C. position 50 nm or less in Shear strain LifeNo. (mm) (Mpa) (2) (2) (mm) or higher Nitriding (HV) dia. (per μm ²)(×10⁻⁴) (×10⁶) 1 4.4 1800 2229 1829 0.15 not applied not applied — 208.6 7.7 2 3.5 1960 2250 1850 0.15 not applied not applied — 19 4.2 7.7 33.5 1960 2250 1850 0.23 not applied not applied — 20 4.1 9.6 4 3.5 19602250 1850 0.23 applied not applied — 20 2.5 9.6 5 3.5 1960 2250 18500.23 applied applied 760 23 2.8 10.1  6 2.2 2113 2359 1959 0.23 appliedapplied 805 16 2.2 15.5  7 3.5 1890 2250 1850 0.24 applied not applied —38 3.1 7.5 8 3.5 1947 2250 1850 0.22 applied not applied — 17 2.5 7.8 93.5 1875 2250 1850 0.23 applied not applied — 25 3.0 10.0  10 3.5 20452250 1850 0.20 applied not applied —  5 7.9 8.3 11 3.5 1975 2250 18500.18 applied not applied — 45 1.7 13.5  12 3.5 1907 2250 1850 0.21applied not applied — 52 1.9 11.0  13 3.5 1881 2250 1850 0.23 notapplied not applied — 15 7.2  0.96 14 3.5 1915 2250 1850 0.21 notapplied not applied —  6 6.8 2.1 15 3.5 1981 2250 1850 0.24 not appliednot applied — 26 4.6 1.9 16 3.5 1895 2250 1850 0.22 not applied notapplied — 33 7.8 2.3 17 3.5 1925 2250 1850 0.22 not applied not applied— 15 10.2  5.0

The following matters have been elucidated based on the results of theabove experiments. First, although Example No. 1 satisfies theexpression (1), the tensile strength TS of the hard-drawn wire ofExample No. 1 is lower than the value in the right object of theexpression (2) because the reduction of area of the wire at the time ofdrawing is low. Consequently, the spring of Example No. 1 has adeteriorated sag resistance compared to the springs of the otherExamples Nos. 2 to 12. The fatigue life of the spring of Example No. 1is as long as substantially equivalent to the springs of the otherExamples Nos. 2 to 12.

Examples Nos. 2 to 9, 11, and 12 all satisfy the expressions (1) and(2), and satisfy the requirement regarding the number of precipitantseach having a diameter of 50 μm or less. The springs of Examples Nos. 2to 9, 11, and 12 show excellent fatigue life and sag resistance. Amongthese Examples, the spring of Example No. 2 differs from the spring ofExample No. 3 in the aspect of shot-peening condition (namely, the shotused in the first time of shot-peening of the spring of Example No. 2has a smaller size than that in the first time of shot-peening of thespring of Example No. 3) despite the fact that Example No. 3 used thesame kind of steel material. As a result of this difference, the springof Example No. 2 has a shorter distance to the crossing point than thespring of Example No. 3, and accordingly, the springs of Examples Nos. 2and 3 have substantially the same level of sag resistance despite thefact that the fatigue life of the spring of Example No. 2 is shorterthan that of Example No. 3.

The spring of Example No. 4 was obtained by applying a stress to thespring at 120° C. or higher, whereas the spring of Example No. 3 wasobtained without applying a stress at 120° C. or higher although thespring of Example No. 3 was obtained without improved sag resistance,the fatigue life thereof is substantially the same as the spring ofExample No. 3. The spring of Example No. 5 was obtained by applying anitriding process, whereas the spring of Example No. 4 was obtainedwithout applying a nitriding process. Although the springs of ExamplesNos. 4 and 5 have substantially the same sag resistance, the spring ofExample No. 5 has an improved fatigue life compared to the spring ofExample No. 4. Example No. 6 is different from Example No. 5 in thedegree of drawing and the diameter of wire, although the springs ofExamples Nos. 5 and 6 are substantially the same in the composition ofthe steel material and the kinds of treatment. As a result of thesedifferences, Example No. 6 has such a high tensile strength as 2113 MPa,as well as improved fatigue life. Example No. 7 uses a steel materialhaving a relatively low carbon content, whereas Example No. 8 uses asteel material having a relatively high carbon content. The springs ofboth Examples Nos. 7 and 8 have excellent sag resistance and fatigueproperties. Examples Nos. 11 and 12 have a relatively high content inchromium and vanadium, and have an increased number of precipitantshaving a diameter of 50 μm or less, as well as improved sag resistanceand fatigue life.

Example No. 10 satisfies both the expressions (1) and (2). However, thecooling rate until the temperature of the wire rod reaches 400° C. afterisothermal transformation at the time of patenting is fast and theamount of precipitants decreases with respect to Example No. 10. As aresult, the number of precipitants having a diameter of 50 μm or lessper unit area is less than ten with respect to Example No. 10. As aresult, the spring of Example No. 10 has a slightly deteriorated sagresistance compared to the springs of Examples Nos. 2 through 9.However, the fatigue life of the spring of Example No. 10 is as long asthe other Examples Nos. 1 to 9, 11, and 12.

Compared to the above Examples Nos. 1 to 12, Examples Nos. 13 through 17are comparative examples. None of these comparative examples satisfy atleast one of the requirements defined in this invention. As a result ofthe experiments, it was proved that Examples Nos. 13 through 17 have adeteriorated characteristic with respect to at least one of theproperties found in Examples Nos. 1 through 12. The chemical compositionof the steel material of Example No. 13 is substantially the same asExamples Nos. 1 through 6. However, since the wire rod of Example No. 13was not subjected to cooling by gas at the time of patenting,pro-eutectoid ferrite was generated on the wire rod of Example No. 13,and the pearlite area ratio of the wire rod of Example No. 13 waslowered than the range defined in this invention. As a result, thefatigue life of the spring of Example No. 13 is remarkably lower thanthe spring of Example No. 7 although the wire of Example No. 13 hassubstantially the same level of tensile strength TS as Example No. 7.

The wire rod of Example No. 14 is made of steel in conformance with thecriteria of JIS G3502-SWRS92B. However, the spring of Example No. 14 hasdeteriorated sag resistance and fatigue life compared to the springs ofExamples Nos. 1 to 12. It is conceived that the reason for such a shortfatigue life of the spring of Example No. 14 results from the fact thatusing the above steel having a higher carbon content resultantly raisesdefect sensitivity and leads to earlier formation of fatigue startpoint. Further, it is conceived that the reason for the lowered sagresistance of the spring of Example No. 14 is due to less content insilicon, chromium, vanadium, etc.

The steel material of Example No. 15 contains a large amount of silicon,and chromium and vanadium are also contained therein. However, since thesteel material having high carbon content is used to produce the springof Example No. 15, the fatigue life of the spring of Example No. 15 isshort although sag resistance thereof is good.

The spring of Example No. 16 contains less amount of silicon compared tothe spring of Example No. 115 (sic). Accordingly, the spring of ExampleNo. 16 has deteriorated sag resistance compared to the spring of ExampleNo. 15. The steel material of Example No. 17 has carbon content in therange defined in this invention. However, the steel material of ExampleNo. 17 contains slightly less amount of silicon. Accordingly, althoughthe fatigue property of the spring of Example No. 17 is slightly betterthan the springs of Examples Nos. 14 to 16, the spring of Example No. 17does not have a fatigue property as high as the springs of the inventiveexamples, and sag resistance thereof is remarkably lowered compared tothe springs of the inventive examples.

Industrial Applicability

This invention is constructed as mentioned above. According to thisinvention, realized are a steel wire rod used for producing hard-drawnsprings capable of exhibiting fatigue strength and sag resistanceequivalent to or higher than springs using an oil-tempered wire, a wirefor such hard-drawn springs, such hard-drawn springs, and a usefulmethod for producing such hard-drawn springs at a low cost.

1. A steel wire rod for hard-drawn springs which contains carbon in arange from 0.5 to less than 0.7 mass %, silicon in a range from 1.4 to2.5 mass %, manganese in a range from 0.5 to 1.5 mass %, chromium in arange from 0.05 to 2.0 mass %, and vanadium in a range from 0.05 to 0.40mass %, and has an area ratio Rp with respect to pearlite whichsatisfies the mathematical expression (1):Rp(area %)≧55×[C]+61  (1) where [C] denotes the content (mass %) ofcarbon, and wherein ferrite in lamellar in which the number of carbideand carbonitride of vanadium and chromium, complex carbide and complexcarbonitride of vanadium and chromium each having a diameter of 50 nm orless in terms of a hypothetical circle, is not smaller than ten per μm².2. The steel wire rod according to claim 1, wherein the steel wire rodfurther contains nickel in a range from 0.05 to 0.5 mass %.
 3. A wirefor hard-drawn springs which contains carbon in a range from 0.5 to lessthan 0.7 mass %, silicon in a range from 1.4 to 2.5 mass %, manganese ina range from 0.5 to 1.5 mass %, chromium in a range from 0.05 to 2.0mass %, and vanadium in a range from 0.05 to 0.40 mass %, and has anarea ratio Rp with respect to pearlite which satisfies the mathematicalexpression (1), and has a tensile strength TS of the wire whichsatisfies the mathematical expression (2):Rp(area %)≧55×[C]+61  (1) where [C] denotes the content (mass %) ofcarbon,−13.1d ³+160d ²−671d+3200≧T S(MPa)≧−13.1d ³+160d ²−671 d+2800  (2) whered is a diameter (mm) of the wire which satisfies the expression[1.0≦d≦9.0].
 4. The wire according to claim 3, wherein the wire furthercontains nickel in a range from 0.05 to 0.5 mass %.
 5. A hard-drawnspring producible by using the steel wire rod of claim
 1. 6. Ahard-drawn spring producible by using the wire of claim
 3. 7. Thehard-drawn spring according to claim 5, wherein a residual stress of thespring is changed from a compression to a tension at a depth of 0.05 mmor more from an inner surface of the spring.
 8. The hard-drawn springaccording to claim 6, wherein a residual stress of the spring is changedfrom a compression to a tension at a depth of 0.05 mm or more from aninner surface of the spring.
 9. The hard-drawn spring according to claim7, wherein the residual stress of the spring is changed from thecompression to the tension at the depth of 0.15 mm or more from theinner surface of the spring.
 10. The hard-drawn spring according toclaim 8, wherein the residual stress of the spring is changed from thecompression to the tension at the depth of 0.15 mm or more from theinner surface of the spring.
 11. The hard-drawn spring according toclaim 5, wherein the spring is applied with a nitriding process on asurface thereof.
 12. The hard-drawn spring according to claim 6, whereinthe spring is applied with a nitriding process on a surface thereof. 13.A method for producing the hard-drawn springs of claim 5 comprising thestep of applying a stress τ (MPa) to the spring at a temperature notlower than a room temperature at least once after shot-peening, thestress τ satisfying the mathematical expression (3):τ≧T S(MPa)×0.5  (3) where TS denotes a tensile strength of a wire. 14.The method according to claim 13, wherein the temperature at which thestress τ is applied is 120° C. or higher.
 15. A method for producing thehard-drawn springs of claim 6 comprising the step of applying a stress τ(MPa) to the spring at a temperature not lower than a room temperatureat least once after shot-peening, the stress τ satisfying themathematical expression (3):τ≧T S(MPa)×0.5  (3) where TS denotes a tensile strength of the wire. 16.The method according to claim 15, wherein the temperature at which thestress τ is applied is 120° C. or higher.